The present disclosure relates to a method and apparatus for washing sand that is contained in wastewater. In particular, the present invention relates to a method and apparatus for separating sand, which is used for bedding animals, such as cows, from organic material for easy disposal of the organic materials and the reuse of the sand.
There has been a practice in the dairy industry to use sand as a bedding for animals, such as cows. The use of sand as a bedding material for cows has several advantages over the traditionally used chopped straw, sawdust or wood shavings. Some of the benefits include improved udder health, increased cow comfort, cleaner cows, improved traction and lower cost. One drawback to the use of sand is the significant handling and storage problems associated with the resulting mixture of sand and organic material, such as manure.
After the sand has been used as bedding, a mixture of sand and other particles is collected. This mixture from the floor of a dairy barn, for example, can be made up of (1) sludgy, hard brown organic matter; (2) organic matter in the form of fibrous or seedy undigested feed particles; (3) outside contaminants such as hair, tails, hoof particles, etc., and (4) sand. The object is to separate the sand from all the other constituents so that it is clean enough to reuse.
The related art has shown an assortment of separation systems used in the dairy, mining and petroleum refining industries. Some separation systems, such as screening and dissolved air floatation, are ineffective for use in separating manure and sand. For example, the dissolved air floatation method is ineffective because the minute bubbles are unable to float the large, coarse manure particles to the top of the tank for removal. Screening is also ineffective due to the similarities in the particle size distributions of the bedding sand and the manure.
In one type of separation system, the sand is rinsed together with wastewater to remove the harmful organic material. In this case, the sand is collected in a collecting reservoir with organic material and then pumped into a sand separator. This sand separator consists essentially of a vertical funnel-shaped container having an overflow. The wastewater with the organic material exits the overflow while the sand exits the container through a discharge opening at the bottom of the container. A discharge conveyor, which is typically a screw conveyer, is provided beneath the discharge opening to convey the sand at an upward angle so that the exit of the discharge conveyor is located above the height of the overflow. A rotating stream is created inside the container in which the characteristically lighter inorganic materials are displaced upward while the characteristically heavy sand sinks downward toward the discharge conveyor. The upward movable organic material is removed from the container together with the overflowing wastewater. The sand, which settles in a region at the conveyor bottom, can be removed by the discharge conveyor with the accompanying draining of the sand because the discharge end of the conveyor is located higher than the container overflow. For the discharging of sand, a stepwise drive for the discharge conveyor is recommended to insure sufficient settling time for significant separation of the sand and organic materials after the introduction of the water. A drawback of this method of separating sand consists primarily in that the discharged sand is still loaded to some extent with organic material which excludes a further use of the sand, e.g., as bedding or as a bulk material in construction.
One attempt to correct these problems has been presented in U.S. Pat. No. 5,811,016, issued Sep. 22, 1998, (hereinafter referred to as the '016 patent). The '016 patent describes a method and an apparatus for removing sand from organic material involving a water-filled container in which the organic material flows upward with the rinsing water to an overflow while the sand sinks downward toward a discharge conveyor connected to the container from beneath. The sand is discharged after a certain settling period. The separation of the sand with the organic material was achieved by stirring the settled sand while simultaneous rinsing the sand with fresh water delivered to the container's bottom region.
The above method and apparatus of the '016 patent does have some drawbacks. For example, the sand, which settles in the bottom region of the container, forms a stop for the organic material to prevent the organic material from reaching the inlet of the discharge conveyor. Thus, there is a requirement that the height of the sand deposit in the container bottom region does not fall below a minimum dimension. As a result, to insure the obtainment of such a vertical sand cake, the sand can only be discharged in an amount which at most corresponds to the excess of sand over the minimal height.